This study will provide a scientific understanding of the specific mechanisms of spastic stiff-legged gain or walking, ultimately providing a scaffold on which to critically evaluate and optimize rehabilitation treatments. Many patients with spastic paresis, regardless of the upper motor neuron pathology will recover the ability to walk, but will walk slowly and stiffly. Although spastic gait is the focus of much rehabilitation effort, a detailed understanding of it is lacking. Some or many of the current irreversible surgical and/or rehabilitation interventions may not be appropriate or optimum. Scientific, including biomechanical, evaluation is definitely needed. A clearly identifiable and common abnormality is insufficient bending or flexing of the affected knee in swing, referred to as stiff-legged gait. A pilot electromyographic study of patients with spastic stiff-legged gait suggested three possible causes of stiff-legged gait: (1) inappropriate vastus activity during the pre-swing and/or initial swing phases of the gait cycle, (2) inappropriate hamstring activity during pre-swing and/or initial swing and (3) delayed heel off, suggesting weak gastrocsoleus or calf muscle strength. The goal of this investigation is to perform a detailed scientific study, including electromyographic, kinematic and kinetic measurements, in a larger and diagnostically homogeneous sample of patients with stiff-legged gait to definitively determine whether, and under what biomechanical conditions, the experimental reduction or elimination of inappropriate quadriceps or hamstrings muscle activity, or the experimental enhancement of gastrocsoleus function can cause an overall improvement in gait. The environment in which these studies will be conducted combines a patient population of the largest private, non- profit, rehabilitation hospital in the nation, with a new state-of-the- art laboratory with expert sponsors and experienced support staff. This study should provide useful, specific therapeutic options, based upon valid scientific data, as well as a more comprehensive understanding of spastic gait in general.